1. Field of the Invention
The present invention relates to thermodynamic cycle apparatus and methods, and more particularly to thermodynamic cycle apparatus including compressors and heat pumps and related methods.
2. Description of the Related Art
One type of engine is commonly known as a Stirling engine. The Stirling engine has a fixed mass of a gas inside its chamber, which remains in the chamber during operation of the engine.
At various points in the cycle of the Stirling engine, the fixed mass of gas is alternately heated and cooled, thereby changing its pressure. The pressure increases when the gas is heated, and the pressure decreases when the gas is cooled. The chamber of the engine is contiguous with a piston chamber, so that an increase in pressure drives the piston outward, and a decrease in pressure drives the piston inward. The moving portion of the piston is mechanically linked to a rotating shaft, which in turn drives a generator and produces electrical power; other uses and applications are also possible.
The heating and cooling functions are typically performed by a pair of hot and cold plates, located on opposite sides of the chamber. A displacer that can easily move back and forth inside the chamber forces the gas into contact with one plate while insulating it from the other. Air flows around the perimeter of the displacer, and movement of the displacer from one side to the other requires very little energy.
The hot plate is heated by a generally continuous source of heat, such as a flame, or a solar panel. The cold plate can be at room temperature, or cooled in a continuous manner, such as evaporatively or being located in or near a bath of ice.
An example of a Stirling engine is shown in FIGS. 13 and 14. The hot plate 136 and cold plate 137 are on opposite sides of a chamber 135. The sides adjacent to the hot and cold plates are thermally insulated, so that the heat flowing in and out of the chamber through the sides is minimized. The chamber 135 is filled with a fixed mass of fluid, which may be a gas such as air. A displacer 134 moves within the chamber 135, and forces the gas inside the chamber 135 into thermal contact with one of the plates, while insulating it from the other plate. Air flows around the displacer 134, which requires very little energy to move from one side to the other of the chamber 135.
FIG. 13 shows the Stirling engine in a “hot” position 130, with the displacer 134 held against the cold plate 137 by the displacer actuator 133, so that the gas in the chamber 135 is in thermal contact with the hot plate 136 and is insulated from the cold plate 137. Because there is a fixed amount of gas in the chamber, when the gas absorbs heat from the hot plate 136, it expands through the chamber outlet 138 and drives the piston 139 outward. As a result, the piston actuator 141 drives a crankshaft 132. The crankshaft 132, pivotably attached to the frame 142 by one or more bearings 143, rotates under the influence of the piston actuator 141, and turns generator 131. The generator 131 produces electricity, for use external to the engine 130. The engine 130 may have an optional flywheel 144 for stability.
Note that it takes very little energy to move the displacer 134 inside the chamber 135, so that the displacer actuator 133 can easily be driven by the rotating crankshaft 132 with very little loss. The displacer 134 itself is a lightweight thermal insulator, and it moves relatively freely inside the chamber of the Stirling engine 130. Its primary purpose is not to compress the gas in the chamber 135, but to force the gas into contact with one of the plates while insulating the gas from the other plate. There is room around the perimeter of the displacer 134 for gas to flow, so it requires very little energy to move the displacer 134 from one orientation to the other.
FIG. 14 shows the Stirling engine in the “cold” position 140, where the crankshaft has rotated 180 degrees from the view shown in FIG. 13. The displacer actuator 133 has moved the displacer 134 into contact with the hot plate 136, so that the gas is in thermal contact with the cold plate 137. The cold plate 137 absorbs some heat from the gas, so that the gas cools and, therefore, contracts. The reduction of pressure inside the chamber drives the piston 139 inward, causing the piston actuator 141 to further rotate the crankshaft 132.
For each rotation of the crankshaft 132, the engine passes continuously from the “hot” state to the “cold” state and back again.
Note that the piston actuator 141 and the displacer actuator 133 typically are out-of-phase, with a value between 0 degrees and 180 degrees.
Conversion of the Stirling engine of FIGS. 13 and 14 to a heat pump is straightforward, requiring replacement of the generator 131 with a motor, and optionally requiring adjustment of the phase between the piston actuator 141 and the displacer actuator 133, so that they are essentially opposite that as drawn in FIGS. 13 and 14. When used as a heat pump, the motor turns the crankshaft 132, driving the piston actuator 141 and, in turn, the piston 139. For heat pump operation, the chamber is forcibly expanded when the gas is in contact with the cold plate, and the chamber is forcibly compressed when the gas is in contact with the hot plate.
For one part of the heat pump cycle, the piston 139 is driven outward by the piston actuator, decreasing the pressure of the fixed amount of gas inside the chamber 135. The gas is therefore cooled, and is cooled below the temperature of the cold plate 137. The gas is then brought into thermal contact with the cold plate 137, and heat flows from the cold plate 137 into the gas, making the cold plate 137 even colder.
In the next part of the heat pump cycle, the piston is driven inward by the piston actuator 141, increasing the pressure and, therefore, the temperature of the gas. The temperature of the heated gas is greater than that of the hot plate 136. The gas is then brought into thermal contact with the hot plate 136, and heat flows from the gas into the hot plate 136, making the hot plate 136 even hotter.
These two parts of the cycle then repeat, thereby converting a mechanical energy supplied by the motor to a transfer of heat from a cold body to a hot body.
The Stirling engine has many advantages over other types of engines. For instance, there is a fixed amount of gas sealed inside the Stirling engine, which never leaves the engine. The heat source may be continuous, so that the amount of exhaust fumes is much less than for comparable internal or external combustion engines. Because the gas is sealed inside the chamber, environmentally risky materials may be used without risk of contaminating the surroundings. Also, a Stirling engine uses an external heat source, which could be a continuously-burning flame, solar energy, or a variety of others. Unlike an internal combustion engine, no explosions take place, so operation of a Stirling engine is typically very quiet.
There are drawbacks, though, to existing engines and heat pumps based on the Stirling engine of FIGS. 13 and 14. For instance, the gas-filled chamber is coupled to a mechanical piston. Mechanical pistons are inherently inefficient, in that they have some amount of frictional losses. In a piston, one solid object moves against another solid object while maintaining a seal between them, and motion of one solid against another invariably has a frictional loss associated with it.
Accordingly, there exists a need for engines and heat pumps, and generally for thermodynamic cycle apparatus, that overcome the inherent losses caused by friction.